THEIR GENERAL CHARACTER AND STRUCTURE n 



Occupying as they do the lowest place in modern classifi- 

 cation of vertebrate animals possessing a skull and brain 

 General (Craniata), the structure of fishes generally is of a 

 structure. more plastic nature than that of more highly 

 organised creatures. This not only brings about extreme, and 

 often fantastic, modification of form in compliance with 

 acquired habit and physical environment, but tends in 

 certain genera to render the separation of species from mere 

 trivial, local, or temporary varieties exceedingly difficult and 

 dubious.* 



For the central type of the class it is permissible to take 

 the most highly organised of British fresh-water fish, the 

 perch, although the superior swiftness and dimensions of the 

 salmon, and its simpler outline, perhaps render it more satis- 

 factory for that purpose. However, having borrowed Dr. 

 Giinther's excellent figure of the perch's skeleton to illustrate 

 this chapter, let us take that fish as the standard of the class. 

 Here we find an animal, whereof the specific gravity is greater 

 than the water, suspended without apparent effort in that 

 medium in a horizontal position, but with its sides vertical, 

 moving in any direction with a minimum of friction at various 

 degrees of speed. The first question suggesting itself is, 

 Why does not the fish, being heavier than the water, sink 

 to the bottom ? 



The explanation is not to be found complete in the 



external structure of the animal, but involves notice of 



The air- an internal organ peculiar to some, but not to all 



bladder, fishes namely, the air-bladder. This consists of a 



sac placed in the abdominal cavity, and is considered to be a 



rudimental form of the lungs of higher animals. This bladder, 



which in some kinds of fish is connected with the pharynx by 



a pneumatic duct, is entirely closed in the perch, at least in 



* Among British fishes this is notably the case with the Salmonidce, 

 and will be more fully discussed in the chapter dealing with that 

 family. 



